As practiced commercially, fused silica optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates and windows, are typically manufactured from bulk pieces of fused silica made in large production furnaces. Bulk pieces of fused silica manufactured in large production furnaces are known in the art as boules or ingots. Blanks are cut from boules or ingots, and finished optical members are manufactured from glass blanks, utilizing manufacturing steps that may include, but are not limited to, cutting, polishing, and/or coating pieces of glass from a blank. Many of these optical members are used in various apparatus employed in environments where they are exposed to ultraviolet light having a wavelength of about 360 nm or less, for example, an excimer laser beam or some other ultraviolet laser beam. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser fabrication equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
As the energy and pulse rate of lasers increase, the optical members which are used in conjunction with such lasers, are exposed to increased levels of laser radiation. Fused silica members have become widely used as the manufacturing material of choice for optical members in such laser-based optical systems due to their excellent optical properties and resistance to laser induced damage.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength excimer lasers operating in the UV and deep UV (DUV) and vacuum UV wavelength ranges, which includes, but are not limited to, lasers operating at about 248 nm, 193 nm, 157 nm and even shorter wavelengths. Excimer laser systems are popular in microlithography applications, and the shortened wavelengths allow for increased feature resolution and thus line densities in the manufacturing of integrated circuits and microchips, which enables the manufacture of circuits having decreased feature sizes. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies in the beam due to the fact that each individual photon is of higher energy. In such excimer laser systems, fused silica optics are exposed to high energy photon irradiation levels for prolonged periods of time resulting in the degradation of the optical properties of the optical members.
It is known that such laser induced degradation adversely affects the optical properties and performance of the fused silica optics by decreasing light transmission levels, discoloring the glass, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Over the years, many methods have been suggested for improving the optical damage resistance of fused silica glass. It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting process, plasma CVD process, electrical fusing of quartz crystal powder, and other methods, are susceptible to laser damage to various degrees.
A common suggestion has been to increase the OH content of such glass to a high level. For example, Escher, G. C., KrF Laser Induced Color Centers In Commercial Fused Silicas, SPIE Vol. 998, Excimer Beam Applications, pp. 30-37 (1988), confirms that defect generation rate is dependent upon the fused silica OH content, and that “wet” silica is the material of choice for KrF applications. Specifically, they note that high OH content silica is more damage resistant than low OH silica.
U.S. Pat. No. 5,086,352 and the related U.S. Pat. No. 5,325,230 has also disclosed that the ability to resist optical deterioration from exposure to a short wavelength ultraviolet laser beam depends on the OH group content in the presence of hydrogen. Specifically, these references show that for high purity silica glass having low OH content, KrF excimer laser durability is poor. Thus, they suggest an OH content of at least 50 ppm. Similarly, Yamagata, S., Improvement of Excimer Laser Durability of Silica Glass, Transactions of the Materials Research Society of Japan, Vol. 8, pp. 82-96 (1992), discloses the effect of dissolved hydrogen on fluorescence emission behavior and the degradation of transmission under irradiation of KrF excimer laser ray for high purity silica glass containing OH groups up to 750 ppm by weight such as those synthesized from high purity silicon tetrachloride by the oxygen flame hydrolysis method.
Others have also suggested methods of increasing the optical durability of fused silica. For example, Faile, S. P., and Roy, D. M., Mechanism of Color Center Destruction in Hydrogen Impregnated Radiation Resistant Glasses, Materials Research Bull., Vol. 5, pp. 385-390 (1970), have disclosed that hydrogen-impregnated glasses tend to resist gamma ray-induced radiation. Japanese Patent Abstract 40-10228 discloses a process by which quartz glass article made by melting is heated at about 400 to 1000° C. in an atmosphere containing hydrogen to prevent colorization due to the influence of ionizing radiation (solarization). Similarly, Japanese Patent Abstract 39-23850 discloses that the transmittance of UV light by silica glass can be improved by heat treating the glass in a hydrogen atmosphere at 950 to 1400° C. followed by heat treatment in an oxygen atmosphere at the same temperature range.
Shelby, J. E., Radiation Effects in Hydrogen-impregnated Vitreous Silica, J. Applied Physics, Vol. 50, No. 5, pp. 3702-06 (1979), suggests that irradiation of hydrogen-impregnated vitreous silica suppresses the formation of optical defects, but that hydrogen impregnation also results in the formation of large quantities of bound hydroxyl and hydride, and also results in the expansion or decrease in density of the glass.
Recently, U.S. Pat. No. 5,410,428 has disclosed a method of preventing induced optical degradation by a complicated combination of treatment processes and compositional manipulations of the fused silica members to achieve a particular hydrogen concentration and refractive index, in order to improve resistance to UV laser light degradation. It is suggested that under such UV irradiation some chemical bonds between Silicon arid oxygen in the network structure of the fused silica is generally broken and then rejoins with other structures resulting in an increased local density and an increased local refractive index of the fused silica at the target area.
More recently, U.S. Pat. No. 5,616,159 to Araujo et al. disclosed a high purity fused silica having high resistance to optical damage up to 107 pulses (350 mJ/cm2/pulse) at the laser wavelength of 248 nm and a method for making such glass. The composition disclosed in Araujo et al. comprises at least 50 ppm OH and has a concentration of H2 greater than 1×1018 molecules/cm3.
While the above suggested methods are at least partially effective in reducing the absorption induced at 215 and 260 nm, there has been little or no suggestion for addressing optical damage caused by radiation-induced compaction resulting from prolonged exposure to excimer lasers. And given the semiconductor industry's reliance on excimer lasers and materials that transmit that energy to make integrated circuit chips and other products and the constant drive towards decreased line width and the necessary wavelength of the incident light and the resultant increase in the laser energy level, it follows that the fused silica material requirements become much more stringent. As such, there continues to be a need for more improved fused silica glasses, particularly fused silica material which is as inert as possible with respect to the incident light energy and thus exhibiting increased resistance to optical damage during prolonged exposure to ultraviolet laser radiation, in particular, resistance to optical damage associated with prolonged exposure to UV radiation caused by 193 and 248 nm excimer lasers.
It is important that the fused silica materials used for elements in the light path in precision optical devices and applications have high refractive index homogeneity. However, unfortunately, depending on the method of production of the fused silica material, refractive index variation in the materials along the light path or transverse to the light path tends to occur. The variation may be present at high frequency and/or low frequency. A typical form of high frequency index variation is striae. Irregular and unpredictable refractive index variation in an optical member in directions perpendicular to the optical axis is particularly detrimental and undesirable. Therefore, measures have to be taken in the production of fused silica materials to improve the refractive index homogeneity.
In the prior art, various methods have been disclosed and suggested to improve the refractive index homogeneity of the fused silica glass boule produced. For example, United States Patent Application Publication No. 2003/0,139,277 A1 discloses that doping aluminum into the fused silica boule can improve the axial refractive index homogeneity. U.S. Pat. No. 6,698,248 discloses an improved furnace design where the distance between the burners and the soot collecting surface remains substantially constant that enhances the axial refractive index homogeneity of the fused silica boule produced in the furnace. Other methods such as oscillating the soot collecting surface have been disclosed and used in the commercial production of fused silica boules in order to improve the refractive index homogeneity, in addition to other optical and physical properties.
However, all these approaches were adopted in the context of producing fused silica boules in a direct-deposit furnace exemplified in FIG. 1 of United States Patent Application Publication No. 2003/0,139,277. In a direct-deposit furnace, silica soot particles produced or provided are collected at a high temperature on a collecting surface, where they are consolidated to form a transparent fused silica boule. Therefore, in this fused silica production process, soot particle deposition and consolidated glass formation are carried out in a single step in a single furnace.
Another approach of forming fused silica glass body involves a two step process. First, silica soot particles are formed and deposited on a soot collecting surface to form a porous silica body. In a second step, after optional further treatment, the porous silica body is consolidated into a transparent glass body by sintering at a high temperature. Unique issues relating to the control of refractive index uniformity in this process have arisen. For example, it has been discovered that the refractive index in a plane transverse to the optical axis may vary to an unacceptable level.
Therefore, there exists a need for an improved process for producing high purity synthetic fused silica materials and such materials having improved optical performance. The present invention satisfies this need.